1. Field of the Invention
The present invention relates to a cellular communication system, more particularly, to a method for controlling an idle handoff in a cellular communication system.
2. Prior Art
In a cellular communication environment, when a mobile station moves from a first cell to a second cell, it is necessary to transfer the mobile station's RF communication link with the first cell to the second cell. This operation is commonly called handoff. An idle handoff occurs when a mobile station such as a vehicle mounted telephone, a portable telephone, or a personal communication system handset has moved from the coverage area of one base station into the coverage area of another base station during the mobile station idle state.
If the mobile station detects a pilot channel signal from another base station, that is sufficiently stronger than that of the current base station, the mobile station determines that an idle handoff should occur. In the mobile station idle state, the mobile station continuously searches for the strongest pilot channel signal on the current code division multiple access (CDMA) frequency assignment whenever it monitors the paging channel. If the mobile station determines that one of the neighbor set or remaining set pilot channel signals is sufficiently stronger than the pilot channel of the active set, the mobile station should perform an idle handoff.
U.S. Pat. No. 5,436,956 (issued to Hirofumi Shiotsuki et al. on Jul. 25, 1995) discloses a method and arrangement for reducing the number of handoff requests in a cellular mobile communications system.
FIG. 1 illustrates a conventional idle handoff controlling method in a cellular communication system. In step S101, when a user powers on a mobile station(not shown), the mobile station searches for a first pilot channel having the strongest strength on a current CDMA frequency assignment transmitted from one of the plurality of base stations as an active base station and measures a strength A of the first pilot channel searched. That is, the radios of received pilot energy per chip, Ec, to total received spectral density(noise and signals), It. That is, A=Ec/It.
In step S102, the mobile station searches for a second pilot channel transmitted from one of the plurality of neighbor base stations (not shown) and measures a strength B of the second pilot channel searched. B=Ec1/It1. In step S103, the mobile station subtracts the first pilot channel strength A with respect to the active base station from the second pilot channel strength B with respect to the neighbor base station which are stored in the memory 206 to obtain a strength difference D. That is, D=B-A.
In step S104, the mobile station judges whether or not the strength difference D is greater than a threshold value C.sub.TH between 2dB and 3dB. As the result of the judgement in step S104, when the strength difference D is greater than the threshold value C.sub.TH, the mobile station starts to search for and monitor a paging channel of the neighbor base station so that the mobile station performs an idle handoff(step S105). Thus, the mobile station can communicate with other mobile stations (not shown) located in a service area of the neighboring base station. When it is judged that the strength difference D is less than or equal to the threshold value C.sub.TH, the routine returns to step S101 and the above process with respect to other neighbor base stations except the neighbor base station already processed repeats.
In the conventional idle handoff controlling method, the threshold value is a fixed value from 2dB to 3dB. When the second pilot channel transmitted from one of the plurality of base stations is momentarily received stronger than the first pilot channel transmitted from an active base station, even when the strength of the first pilot channel is sufficiently strong, the mobile station should perform the idle handoff.
Accordingly, waste of a battery of the mobile station increases and arrival rate of the mobile station lowers due to frequent idle handoff. When the mobile station moves to another area, the mobile station should periodically provide the information about the position thereof to the base station in order to receive a terminated call from a base station. Thus, a load of a reverse control channel is increased and communication efficiency of a reverse control channel becomes lower. Particularly, when a paging zone is changed, arrival rate of the mobile station lowers due to frequent idle handoff. It is therefore highly desirable to reduce the number of handoffs in order to provide the service of good quality.